1. Field of the Invention
The present invention relates to a stopping position control device which controls the stopping of an object at a predetermined position (hereinafter referred to as stopped position).
2. Description of the Related Art
Recently, especially with regard to compact cameras, many cameras equipped with a power zoom lens have been developed. When a zoom lens is utilized usually the full-opened aperture value is changed in accordance with a focal length of the lens, and accordingly, the full-opened aperture value must be input to the camera control mechanism to carry out an automatic exposure control with a zoom lens. Further, in a camera in which an exposure program, for example, is changed in accordance with a focal length, data relating to the focal length of the lens must be input to the camera control mechanism.
Therefore, in a conventional device, the position of a zoom ring, which causes zoom lens groups (a variator lens and a compensator lens) to move relatively close to and apart from each other, is sensed and a corresponding full-opened aperture value and focal length of the lens are obtained from the lens position data. It is noted that no particular problem arises if the full-opened aperture value and the focal length of the lens are not continuously obtained, and thus this data can be obtained at predetermined intervals.
Two methods of sensing a stopped position of the zoom ring are known; one in which a range of movement of the zoom ring is divided into a plurality of sections, each section is given a different code, and the codes are discriminated by a code discrimating mechanism, and another in which periodically varied codes are given to the entire range of movement of the zoom ring and a code change counting mechanism is used to count the number of changes of the codes from the base position thereof.
In the former method, digital codes composed of a combination of electro-conductive elements and insulating elements are provided at each division of the range of movement along a direction of movement of the zoom ring. The digital codes are read by a brush kept in slidable contact with the electro-conductive element and the insulating element at each bit, respectively.
In a position sensing mechanism using such codes and a brush, however, due to deterioration with the passage of time, for example, by rust or transformation of the electro-conductive parts, or due to the presence of foreign matter between the code plate and the brush, a poor contact therebetween is obtained, and accordingly, reading errors easily occur.
On the other hand, an optical method of code counting is known in which an optical stripe pattern composed of high reflectance elements and low reflectance elements are arranged alternately along a direction of movement of a cam ring. The number of changes of the optical stripe pattern are read and counted by an optical mechanism (a photo reflector) provided at a fixed position on a camera body, whereby a position of the zoom ring is sensed according to the count of the number of changes from the base position thereof.
In this stopping position sensing mechanism, however, if the zoom ring stops when a beam from the photo reflector is at a boundary between the stripes of the pattern, one of two states occur, i.e., a change of a stripe is read and the zoom ring then stopped, or a change of stripe is not read and the zoom ring is stopped. Also, when the movement of the zoom ring resumes after the zoom ring has been stopped on a stripe boundary, a change of a stripe may or may not be read.
This uncertainty, i.e., the reading or the not reading of the change, is a reading error which is accumulated, whereby the error in the full-opened aperture value is increased. Further, in such a case, to prevent an overrun of the zoom ring, a limit switch must be provided at a position opposite to the base position of the zoom ring, and this provision of the limit switch inevitably increases the cost of the camera.
In the electrical reading mechanism described above, the electro-conductive elements and the insulating elements can be replaced by an optical pattern including two kinds of elements having different optical reflectances, and the brush replaced by a plurality of photo reflectors for discriminating the optical pattern of each bit.
A photo reflector, however, is bulkier than a brush, and therefore, the size of the camera must be increased to enable the accommodation therein of a plurality of photo reflectors, and thus the cost thereof is inevitably increased.